1. Field of the Invention
This invention relates to freeze-dried animal skin tissue products, especially for therapeutic use on wounds, and to methods of producing such products. As used herein "skin" and "tissue" refer to whole or split thickness animal skin, porcine or human, unless otherwise indicated by context.
2. Description of the Prior Art
Generally, the lyophilization of skin tissue is accomplished by introducing frozen skin into a freeze-dry apparatus and performing a freeze-dry cycle, which may be summarized as drawing a vacuum, heating the tissue without thawing it to sublime the ice, removing the ice from proximity to the tissue, and gradually raising the temperature of the tissue to ambient temperature as it dries.
The prior art freeze-drying method as it pertains to skin for therapeutic use on humans uses skin tissue which first has been treated by other processes which remove dirt, debris, hair, and unwanted layers of the full thickness skin. Typically the skin is frozen, then irradiated with gamma rays, and stored temporarily in the frozen state to await freeze-drying. Later it is loaded by batches into a freeze-dry apparatus and the ice is sublimed and removed from the presence of the skin by warming the skin without thawing as it is gradually brought to room temperature. The dried product has about 0.5-5 weight % water. Descriptions of the details of representative prior art processes may be found in publications such as British Journal of Hospital Medicine, April 1971; "The Long-Term Storage of Skin," by T. D. Cochrane and John Watson; and "Description of Current Tissue Bank Methods of Skin Preservation," by Vernon P. Perry, Cryobiology, Vol. 3, No. 2, 1966.
The advantage of freeze-dried skin is that it can be stored for long periods of time at room temperatures until need arises. In this way therapeutic skin, for example, can be accumulated in quantities more conveniently in other forms such as frozen and fresh.
When needed the skin tissue can be rehydrated with water to a condition approaching that of fresh skin.
However, a problem in the prior art has been that the tissue produced by the prior art process approaches that of fresh skin on reconstitution, but not as closely as desired. The extent of reconstitution determines the ability of the skin to conform to the patient's affected part. The common therapeutic use of skin, as for example split thickness porcine skin, is as a bandage for extensive burn wounds. Here the suppleness, texture, and moistness of the skin, all of which are functions of the degree of reconstitution of it, are important to obtaining conformity to the body and mechanical adherence to the wound.
The more completely reconstituted the freeze-dried skin tissue, the more useful it is. Inasmuch as the prior art freeze-dried skin tissue falls short of the optimum product, a freeze-dried skin tissue which more fully reconstitutes is much desired.
Another problem in use centers around the time required for reconstitution of freeze-dried skin. In treating burns, time is a factor because the patient's essential bodily fluids are lost through the wound and the body is open to invasion by infectious organisms, at a time when the body defences are debilitated by shock. Prior art freeze-dried skin requires from one-half hour to an hour to reconstitute. Any reduction of this time requirement is an enormous benefit and much desired.